Together with multimedia, personal communications allowing people to communicate with anyone by any medium, anywhere and at any time represent one of the most prospective fields of strong potential. Personal communications are included in both concepts of wired and wireless telecommunications. Great hopes are placed on wireless communications. In wireless communications, portable phones have been spread throughout in developed countries so rapidly that the conventional analog communication system could not satisfy the increasing demand. Accordingly, digital systems which are excellent in capacity of accommodating subscribers, communication costs, secrecy and a variety of communications are nowadays prevailing over the analog systems. There are two digital methods--TDMA (Time-Division Multiple Access) and CDMA (Code-Division Multiple Access). The CDMA is more attractive to users since it can accommodate more subscribers than the TDMA method.
The CDMA system uses the spread-spectrum technique that transmits a signal carrying an information spread over a bandwidth much wider than the information itself-occupied bandwidth by using spread coding system. There are two methods of spread-spectrum technique: one is direct-spreading (DS) method and the other is frequency hopping (FH) method. The cellular telephone system uses the DS method. With the CDMA using the DS technique, each mobile station may spread spectra by using different spread code and transmit the coded channel signal multiplied in the same bandwidth. On the other hand, The receiving side demodulates only the desired signal by despreading with the same spread code of the desired receive channel, separating other interference signals as wide-band noises. Thus, the CDMA system using the spread-spectrum technique offers a number of advantages: it can accommodate a large number of subscribers, allow asynchronous accessing, bear against multi-path fading, enable soft-hand-off by using a plurality of rake-receivers and have a high durability against interferences from other systems and a high secrecy property. On the other hand, mobile communications may usually have interference signals much larger than desired receive signals, causing impairment of a carrier-to-noise (C/N) ratio of a signal after despreading, i.e., making it impossible to conduct correct communications. Accordingly, the CDMA system requires fine control of transmission power.
Thus, the digital communication systems are replacing the analog communication systems since the former systems have excellent features over the latter. However, digital and analog systems coexist while shifting to the digital communications. Mobile stations must be dual-mode terminals adapted to analog and digital systems.
In the dual-mode cellular telephone systems adapted to the analog system and the digital CDMA system, analog and digital systems can work in different frequency spectra in the same area or neighboring areas. However, the following four kinds of interference modes may occur between the base stations and mobile stations of two different systems.
(1) Interference from the analog base station to the digital CDMA mobile station; PA1 (2) Interference from the digital CDMA base station to the analog mobile station; PA1 (3) Interference from the analog mobile station to the digital CDMA base station and PA1 (4) Interference from the digital CDMA mobile station to the analog base station. PA1 (a) An interference by intermodulation distortion caused from non-linearity of a receiver of the CDMA mobile station; PA1 (b) An interference by intermodulation distortion caused from non-linearity of a transmitter system of the analog base station; PA1 (c) An interference by sideband noises of transmission signals from the analog base station.
Among the above, interference mode (1) is the most important. Namely, the interference mode (1) becomes very large at a mobile station working in the digital CDMA mode if the mobile station locates far from the CDMA base station and near to the analog base station. Hereinafter explains a disturbance by interference from an analog base station to a digital CDMA mobile station. The mobile station is communicating in the CDMA mode with a CDMA base station, receiving, at the same time, a number of narrow-band signals as disturbing signals from an analog base station existing in the neighboring area. In this case, the narrow-band analog signals may largely affect the mobile station since they have much a higher level than the desired CDMA-band signal.
Three interference modes may occur as follows:
The interference mode (c) is a phenomenon where sideband noises of an analog signal from an analog base station may fall, as a disturbing signal, into a bandwidth of the CDMA system if the analog transmission channel bandwidth is near the CDMA bandwidth. The effect of this mode is smaller than the modes (a) and (b).
The causes and problems of interference modes (a) and (b) are as follows:
There's a case that a mobile station working in the CDMA mode are receiving a number of analog system signals. When a large-power level narrow-band analog signals (f.sub.1, f.sub.2 and so on) are input to a receiver, a large number of third and fifth intermodulation distortions (2f.sub.1 -f.sub.2, 2f.sub.2 -f.sub.1, 3f.sub.1 -2f.sub.2, 3f.sub.2 -2f.sub.1 and so on) may occur in a receiver due to the non-linearity of its components (e.g., a LNA (Low Noise Amplifier), mixer and so on), producing a large number of narrow-band disturbing signals in a frequency band corresponding to the CDMA band. The produced narrow-band disturbing signals having very high levels results in increasing noise level after despreading the received signal by the despreading portion. Consequently, such a problem arises that the signal may represent erroneous data due to its insufficient carrier-to-noise (C/N) ratio.
An analog base station transmits a large number of narrow-band signals at a time and, therefore, may produce a large number of narrow-band disturbing signals within a frequency band corresponding to a CDMA band due to intermodulation distortions (of 3rd-order, 5th-order and so on) if its transmission system (transmission power amplifier) has non-linearity. Accordingly, the analog transmission system is usually designed so that intermodulation distortion may be satisfactorily small. However, the CDMA mobile station may receive at its receiver a relatively high-level narrow-band disturbing signal while it works near the analog base station. Consequently, a noise level of the received signal is increased after despreading by a despreading portion. An erroneous data may be produced due to insufficient carrier-to-noise ratio. This is a problem to be solved.